Filter cartridges are commonly attached to respirator mask bodies to filter air before it is inhaled by the respirator wearer. Examples of respirator filter cartridges are shown in the following U.S. Pat. No. 7,320,722B2 to Mittelstadt et al., U.S. Pat. No. 7,419,526 to Greer et al., and U.S. Pat. No. 6,277,178 to Holmquist-Brown et al.
Some filter cartridges have an end-of-service-life-indicator, or ESLI, that provides a visible color-change signal to the respirator wearer. This visible signal (sometimes referred to as the “response signal”) indicates when the filter cartridge has met the end of its useful life. Examples of cartridges that use ESLIs are described in the following U.S. Pat. Nos. 6,497,756, 4,530,706, 4,326,514, and 4,154,586. In each of these products, the filter cartridges are provided with a transparent sidewall or a shell through which the ESLI may be viewed. ESLI colorimetric sensors often exhibit a different color, depending on the angle from which the sensor is viewed. When a person does not view the ESLI normal to its positioning on the filter cartridge, there is an opportunity for the viewer to misread the indicator. A misread indicator could cause the wearer of the respirator to replace the filter cartridge too early or too late. Known filter cartridges have allowed the viewer to see the ESLI from angles of about 180° and accordingly have not addressed the issue of potential misread indicators.